You need to reduce the size of the photo.
The easiest way I do that, is I take the picture using the camera on my phone. Then, I send the photo to my email. Usually a box will pop up before I send it, asking what size I want to send. Don’t choose original. I usually choose medium or even small, if I’m planning to post it here.
When you open the email, save the photo, the upload that one to here, as it should be small enough to work.
If you don’t have a smart phone, then I don’t know what to tell you.