i think there is an option to "save for web" in photoshop, which i think allows you to save it to a specific size. But i tend to use photoshop to save the initial image(final version) and then open a different paint program to save that image for the upload,

saved(100% jpeg) image is roughly say 800kb in photoshop, open another paint program, open that saved image, then goto "save as"( i put this new save in my rendo upload folder) and save at say 98%, then click save again(to see if it is small enough) if not then i will save over that at about 95% or whatever!! that way you only end up reducing the image once, as all subsiquente saves are just specifing reduction from that original 800kb image.

hope that makes sense, remember to be under the 512kb for image and under 15kb for thumb to be sure they will upload, if you are exactly 512kb and 15kb they probally won't upload.