For comic I use Medibang paint. The smartphone one.
And for resolution, it's always, always better to have a bigger pixel size regarding the platform you use. You can always resize the picture to fit other platform and it won't be too pixelated (I think). Tapas maximum width is 900+pixel so I would recommend like, 2000px for the width.
tapas resolution limit is 940px wide and up to 4000px long with a file size limit of 2mb so it may be best to work from a larger canvas size like silverraven mentioned and if its an available option on krita a dpi of 300 or more so there's no loss of quality when resizing down to fit tapas' size restrictions
Right now what makes your comic hard to read isn't resolution but some the colouring choices. Many illustrations have characters/objects/situations that share nearly the same values and thus blend into each other. This can make whatever is going on hard to read and lose impact. Especially if you have a form of colour blindness.
If you are not sure if your layout is readable to all forms, you can do a quick check via greyscaling your piece.
As for myself I use GIMP. The comic I'm drawing right now is 3000x4800px 300ppi and I scale it down to a width of 900.
For Detox4 we make it at 1988px X 3056px (we post it somewhere else too and reformat it for tapas) at a 400 DPI. We do 95% of the art on Paint Tool Sai and the rest on photoshop (effects and lettering). Here's an example of how it comes out:
For my comic, I use Firealpaca. I always draw at a higher pixel by pixel ratio then scale down the .png and .jpeg files to 940x1426 px when I upload. I only work in 350 dpi because my comic is in color.
I use GIMP 2.0. I have a vertical scrolling comic so I set my width to 940px(max tapas allowance) and my height varies depending on the panel composition (usually shorter). My DPI I set at 300.
I use CSP for drawing & inking the initial page(plus certain FX) at 600dpi. I then color in Photoshop, so that bumps up the res/pixels even more.
i use medibang paint with a wacom intuos pen small! as for resolutions, i used 800x12800, then i use this website19 to cut it into the sizes needed for both webtoons and tapas (huge lifesaver, trust me)
i used to use 800x20000 if you want something bigger to work with.
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I use Medibang Paint and my wacom to make comics!
My resolutions are 700 x 17920 px. Yes, I know the file is big, but it's pretty easy to cut if you don't have photoshop or any of those fancy programs. Once I finish with my comic, I go on this website called Croppy https://knicknic.github.io/croppy/6 who arranges all my files in order, preparing it for whatever platform you want to post your comic on. They have options for both Tapas and Webtoons 
Hope this helped 
Clip Studio Paint for the line art and Photoshop for the coloring part for Cosmos Song3. Each page dimension size varies but it's always 300 DPI resolution.
I use Clip Studio Paint and I work in 2000px wide x any height that the episode demands. Since I'm doing vertical comics the height size can be insane (the first episode of the webtoon that I'm about to launch has more than 40000px of height!). I had to split it on 4 parts so my laptop stops lagging XD
I like to work in a bigger canvas because after I resize the picture to 900px or 800px wide, the art looks smoother.
I use Clip Studio Paint and Photoshop. My image size is B4 size at 600DPI.
Your images have some pixelation so I would recommend working a bit larger and then scale down for the web and save as a PNG-8 or possibly PNG-24. PNG-8 has less colors/information, but your pages look pretty simple and aren't transparent so that might be enough. I save mine all as PNG-8 because the pages are black and white.
I use the followings
1- CLIPSTUDIO Paint - For basically all the drawings and stuff
2- Photoshop - Color correction and to cut the pannels
3- 3DS Max - For modelling the 3D backgrounds
Sometimes I use things like Substance Painter for especific 3D things like textures. I rely a lot on 3D for optimizing my production time, since I have college+work+freelance to suck my time and I'll be damned if I can't update that comic at least twice a month
Let's resurface this old post!
I sketch & line my B&W comics traditionally and the rest digitally.
I use VueScan to scan, Photoshop as primary (cleaning scans, fixing linearts, color flating+shading, change flats+shades into tones, lettering) And sometimes Krita (for faster color flating+shading, also when delicate lines need to be re-drawn cos' it's better than Photoshop-- at least using from my not-so-powerful tablet pc)
For Resolutions:
I scan my custom 6.6" x 9.3" comicpages, resize and crop into 5.1" x 7.8" 600dpi, or about 3060x4700 px (that's the printing size without bleed areas)
My output dimensions for Tapas are 750x1155 + extended pixel white gaps = 765x1200 px.
I'm a bit meticulous on formating
Programs I use:
MediBang Paint Pro - For sketching and line art.
Corel Painter Essentials 6 - For flat colors, textures, and shadows. I used it for sketching and line art before I switched to MediBang Paint Pro. Didn't really like using this program for line art. 
Adobe Photoshop CS2 - For lettering, outlining speech bubbles, arranging panels, and slicing the comic.
For resolutions, I go for 799px wide and 867px long for each panel or higher than that depending on how much my laptop can handle.
