Because of the way the forum was built, inserting photos 'the Facebook way' takes up server space, so, as this is a free forum, the best way is to sign up to a photo sharing site such as www.dropbox.com (there are others out there).
Once you've started a new thread, or post, and are typing in your text, when you arrive at the bit to insert your photo/image, click on the photo icon in the ribbon (above):
Then go over to your photo sharing website (dropboxt etc) and copy the image 'url' text.
Then paste the url text back into this forum where you left off. In the field 'Image URL'.
Alt text isn't necessarily needed, it's more to help the search engines identify what the photo is 'about' in case the image doesn't load up and leaves a photo placeholder gap instead (is also good practice for disabled people - visually impaired etc who use screen readers) - some people will type in a small description, such as 'PX125 in the driveway' or 'at the Morecambe rally' etc - leave it blank if you don't want to write any text.
and your photo will then display AFTER you have finished creating your thread 'Create Thread' button.