I always work at least 300dpi on a canvas the size of standard paper (8.5 x 11 for my work, as I'm US-based) since I never know when I might want to print something, or if the client wants it printed if I'm doing a commission.
Jpeg's aren't always the best choice for web-display, you get a lot of artifacting/blurriness. And they should only be reserved for images with a lot of range like photographs. For crisp images, png is best, unless the site you're uploading to has file type restrictions.
So another thing that may be causing your loss in image quality is your second step in saving. Each time you save an image in a new program, it compresses it further, deleting digital information, thus leading to quality loss (especially if you're going jpg to jpg). It's best to downsize in the image's native program where you created it in. If you can't, for the first save, save it as a high res file type like TIFF (which is normally recognized by most programs) and then save it into a smaller file type the second time.
EDIT: another rule of thumb I go by is the images I upload to the web are reduced to 72 dpi as it deters art thieves and they still display relatively fine and are still legible.