On computer it's easy
Right click on your photo on your PC and select "open with paint". When paint is open select "resize". A smaller window opens. Select pixels (not percent) and type 650 in the horizontal box - the vertical size will change automatically. Then click ok. Then still in paint go to File - Save as - JPEG. And save it. Important to do save as rather than save, so you keep your original photo and also the one that now has the file size shrunk to fit the forum.
Then in your message on the forum, click on "Go Advanced" (underneath message box). Scroll down to "Manage Attachments" and click on that. Another smaller window opens. Click on "browse" then select the photo on your pc that you have just shrunk to 650 wide. Then click "upload". Wait and it will tell you when the picture has uploaded. You can do this with a maximum of five pictures on one go. When you've finished uploading, close the smaller window then go to the top of the message box where you're typing and wanting to add the photos.
Click on the paperclip and a little drop down box will show the file names of your uploaded photos. Select "Insert all". Before selecting "Insert all" make sure the typing cursor is at the end of the message!
Then click "submit reply" at the bottom of the message box. And your post will appear with the pictures inserted.
You can go up to 750 wide without stretching the forum boxes, but I find 650 is about right. You need to do this with all photos before uploading and attaching them or they're too big to fit in the message box.
Once you've done it a few times it's very quick and easy. You can also insert the photos one at a time, between pieces of writing instead of "insert all".
There is a sticky on how to do this somewhere but I couldn't find it! This assumes you have a Windows PC with the paint program. If you have an apple mac there will be another way of shrinking the file size before uploading but I wouldn't know how.