Showing images in posts takes a bit of bandwidth usage. The larger the image file size, the more bandwidth will be needed to download and display it. Bandwidth might have to paid for by someone somewhere along the line. If this is the case, it could be why the requirement is that images are kept to an acceptable file size.
I note that the recommendation is 50KB but images are showing in the forum that have a file size larger than that. There's one where the size is 88.65KB for instance. No problems with it being seen in the page at that file size. I'm not sure what physical size is allowable for images but the forum software seems happy enough with a size of 600 pixels being the larger dimension.
The trouble is that photos coming straight off a camera can be huge in both file size and physical size ...well over what any forum would allow. A photo editing program will be needed to make it more or less comply with the parameters mentioned above. I think that once an image has been suitably processed, it should upload and display OK.
As well as full image editors, there are also programs that make it possible to right-click on an image file that has come off the camera and then choose a size. Try searching for 'right-click resize image'. It should bring up a program or two that does this.
The usual warning applies to all free software. Choose the 'Custom' or 'Advanced' installation. Read the installation options carefully and deselect any unwanted offers.