If you're using Windows, the "trick" is to resize the photo (then "Save As..." and rename it so you don't lose the original) in a photo manipulation software. I'm on a Linux machine so can't check to see if Microsoft "Paint" can resize, but I think it can. If you find the "resize" function, 600 pixels is about the limit for the wider dimension of an image (with "retain aspect ratio" checked). File size for attaching is, ummm, 150kb, I think.
And you can retry putting it in the original post with the "edit" button in the lower right corner of your post. It allows you to go back and "File Manage" additions.
Sorry... clear as mud, I s'pose.
Google "<span style="font-style: italic">Irfanview</span>" for some good image manipulation software, it's not overly complex and does the job.