Hopefully this helps- but this is how I generally post photos in forums. A lot of the rules are strict as far as image sizes and formats go-so I throw whatever I need to an Imgur and link to the album.
You get full-size photos and the forum page loads much faster for everyone. Also, the forum hosting doesn't have to foot the resource requirements to store them all.
https://imgur.com/uploadIts easy-no need for a log-in, either.
Upload to an album (drag and drop).
Click "Share" to get the link.
Dump link in post.
See photo.
If that's against the forum rules, please reconsider.
I also use a free program call IrfanView.
https://www.irfanview.com/Resizing to hit file size constraints is easy
You can open a picture, choose "Save As", adjust the constraints in the popup and save.
Voila.