The way I do resize is take it from the mobile device through USB to find it in the DCIM folder for the mobile device upload. Then right click and select open with Microsoft Office Picture Manager. Then I click edit, select Large Size 1024 X 768 and hit OK under original size / new size. Then I close the editor and exit MS OPM. A 1 Mega Byte picture is then usually under 200 Kilo Byte after the resizing and works on MB.com
It may be that you want to keep a high resolution picture so in that case your mobile device still has it left in a high resolution. Also keeping another backup copy high resolution on a PC HDD is what I do as well.
If it is Android I saw this site
I never used any of the apps that resize, but another way maybe to see if emailing a photo to yourself on a mobile device automatically resizes smaller for email or has options for size, but I saw no adjustment on the android phone of mine.
Only if you set the size before taking the pictures then that is easy to do in the camera setup. That is not really best if you want a high resolution back up.
Also is sort of funny is the 2nd to last comment and the moderators reply