If your pictures are on a phone, then there are apps that will change the pixel size of the photo. On my iPhone I use a free app called ‘Resize It’, though there are many others.

On a PC, the standard Windows photo viewer has a right-click option to change the picture size, and any decent photo editor should have a similar function.

If you use Tapatalk to access the forum, then that has a built-in resize function.