Mea Culpa

I don't post many photos, but when I do, I use the "Attach Files" button at the lower left of the window where you type in your text. You have the option of posting the full-size image or just a thumbnail; in the case of the latter, readers just click on the thumbnail to see the full-size image. I also use a photo-editing program to reduce the size of the image before posting so the entire image (at least the width) fits within the limits of the area of the forum posts.

What works for me is to click on the little photo icon in the banner that is at the top of the posting box. The photo icon is next to the smiley icon. It is a rectangle with sun and a mountain inside which bytheway looks remarkedly like the Valcarlos route over the Pyrenees.
A little box pops up.
Click in the box.
Two options will appear.
Select the one that says photo library (or something along those lines). It will take you to your photos.
Browse your photos and select the one you want.
Click ‘done’.
By some wonderful @ivar magic, your photo will I think (I could be wrong here) be automatically reduced in pixels and will appear in your post.
Sounds a bit involved but once you’ve done it once or twice, it is quite straighforward.
Best of luck.