You might be better off using a photo edit program rather than "Paint" or a draw program. I use one called PhotoPlus from Serif, as well as their PagePlus pulishing program. In a photo editor, usually you can do two things, "crop" the image to reduce its borders by keeping only what you wanted to see, and secondly, by "resize image" I can set the size, either by physical size, or by # pf pixels. I ususally use the pixels to resize and that way I'm assured to meet the requirements on size. Once I've got the picture the size I want, I can then "export" or "optimize export" in any format, but mostly .jpg. When optimizing I can set the quality % and be assuered that I'm under the 70K limit for here. Usually 80% on a .jpg does not appreciably alter the picture quality, but does reduce the file size greatly. Most of what I post is under 50K and 600 x 400 pixels and usually show a resonable amount of detail. I can do basically the same thing in PagePlus, but I have the additional advantage of adding text and combining more than one picture.
I have several other photo programs, some come with a scanner, others come with the camera, and I'm not sure but one might have come with Windows.
Hope this helps,
Don