When you scan, you can usually select what form of output with your scanner. For posting a pic in forums, jpeg is the standard. For your records you might want to scan in higher resolution (larger files) than you could post in a forum -- because of the usual forum smaller file size limits. But no worries, because you can then take each larger file that you want to post and resize it. There is a simple photo editing program you can download and use online that is free and does some neat stuff -- along with simple stuff like resizing. I use
https://pixlr.com/
It is from a very reliable software developer Autodesk so no worries!
Once you get the program on your device, just open your image, under "fast" scroll down to "Resize" and enter something like 640 or 800 as the largest pixel dimension. Be sure the "aspect ratio" is locked or you will distort the pic, lol. Then "Save as" a new file name. I just add "web" to the existing file name to differentiate from the full-sized original. If you "Save" instead of "Save as" you will lose your original full sized file, so don't do that!!!
I hope this helps