Hi
Upload the photos directly with the post rather than a dropbox download link.
Alternatively, use an image hosting site like imgur which will give you a link. Then embed the link in your post using img tags. Easy way to do that is to copy the link into your post, highlight it, then click the picture button in the toolbar above (button looks like a mini Mona Lisa, and is under the B (bold) button.
Some people use the latest "NOT A PROJECT" thread to upload their photos, then take a copy a link from there to embed in their posts.
Maybe you have a cold solder join or a crack or corrosion in a PCB trace or the CRT socket that briefly comes good - until your contact cleaner fluid dries off. Inspect everything closely and maybe tap/poke gently with a wooden spoon or back of a screwdriver while operating the monitor to see if it temporarily improves.