@marygold Here's a couple articles that will be able to explain more clearly than I can about image size, and ALSO some tricks for avoid blurring when you downsize.
http://www.vsellis.com/understanding-dpi-resolution-and-print-vs-web-images/
https://www.sitepoint.com/resize-an-image-in-photoshop/
It makes sense for Tapastic to have a maximum width that's smaller than print pixel size, since those images would all be a couple 1000 pixels, and take up the entire screen or more on any browser - it works for print, but it's terrible for online reading. That's why there are different industry standards for web and print, and sadly there's no cutting corners when it comes to making a file that will do both - you simply have to resize, and keep different versions of each file (I have a high res folder and a web folder for me comic Meat&Bone).
Another issue you might be running into is if you're saving your files over a certain amount of kilobytes, I'm pretty sure Tapastic will compress them. To avoid this, try saving as a PNG instead of a JPG, and try to keep the kbs low - double check your file before uploading. Once you know what settings work best for your page, you can apply them to future resizing.
Hope I've been helpful!
I fricking laughed XD