Holice - I'm quite good with computers, and I struggled with this a few days ago - and had trouble following some of the instructions in the help folder, so you're not alone!
This is what I did. If you're replying to a thread, you need to choose "Go advanced" when replying. Scroll down and you'll find "managing attachments" and from there can choose your photo. Similarly, if you're starting a new thread, you'll also find a "managing attachments" button below the text box.
You may also need to reduce the size of your photo before uploading it - there are facilities in Windows for doing that -varies a bit from one version to another. In Windows 7, open your photo, choose edit, organise or share, then the edit tag, and you can resize your picture.
When you've clicked on "manage attachments" use Browse to find your photo and upload it.
if it won't upload, the picture may still be too big.
I hope this helps - don't know if it's the "correct" method, but it worked for me!