Hi, Tori! : ) Welcome to the ranks of comic-creators!
Dpi is a kind of tricky thing to wrap your head around, so I'll try to give you a hand!
I don't know what dpi you draw at (I usually do 300-400, since I draw a full-colour comic - when doing black and white or greyscale, 600 or up is a good idea), but a high dpi is really only needed when you're going to print something. When uploading to the web, it's usually a sensible idea to resize it to 72 dpi. It makes for smaller file-sizes (so it's faster to upload, and takes less space on your harddrive) - though I always keep a 300-400 dpi file around for printing.
I draw at 3307 by 4677 pixels at either 300 or 400 dpi, and when downsize to 800 by 1131 pixels for uploading. This means that I lose quality, yes, but since my readers don't zoom in on the page at more than 100%, it still looks fine. (also, it has the added bonus of hiding a bunch of inking and colouring mistakes! XD). Tapastic's maximum width is 940, but you don't need to go that big unless you really want to, as long as the text is readable.
As you can see, my pages still look okay at 72 dpi.