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What program are you using! If it is photobucket, I've found you have to pick the size first, then upload your pictures, otherwise the stay as before. Hope that helps.
Some programs offer no size upload option. Some downsize them all to one size. Photobucket (free) in the upload area has an OPTIONS that allows you to choose the size. If you use it, choose 800 x 600 pixels or smaller. this will keep members from having to scroll right to view all of photo and text.
Before you upload to your photo server, Use the resize function. 640x480 pixels looks good on these forum boards. For current computer displays, around 50% jpg quality is plenty good, and makes a more compressed, smaller file size. Saves you time uploading.
Save-As to a new file name. Never write over your original picture file.