At the bottom of the reply screen are 3 boxes. Click the one that says "switch to full reply screen".

At the bottom of that screen you will se "File Manager" in blue - click it.

When the window opens click "choose file" and then you can just grab a photo from your computer files. When you've done that click "add file". You can add up to three photos following the same process.

When you're done, click "done adding files".

Make sure all the files fit the size limit. You may have to resize some of them. I'm not getting into that !!