There are several variables that can cause issues when posting images to the forum i.e. posting from a computer vs mobile device, image size, image rotation (sideways pictures are very prevalent these days). I'm going to assume you are posting pictures that reside on your computer's hard drive since you mentioned your "computer" in your post. I was about to create a write-up on how to do this but is looks like member powerdriver_1958 has already created an excellent one located in this thread
https://67-72chevytrucks.com/vboard/...d.php?t=627506. I don't agree with the necessity of making your image 640x480, that's pretty small by today's standards and will make some information too small to see or read if you are posting a detailed picture i.e. schematic. I believe the forum will resize an image if it is too large (to an extent) and have never had the forum give me an error when trying to post an image.
The annoying sideways image issue is due to the vBulletin forum software itself not recognizing the image rotation coding applied to an image when a picture is taken with a digital camera or mobile device. It's easy to fix but must be done on a picture-to-picture basis, unless you have access to, and are well versed in the use of a dedicated image manipulation software like Adobe Photoshop. Many of us aren't and we rely on others to correct the issue, or we just say "screw it" and leave them sideways. I fix image rotation issues with Microsoft Paint since it comes pre-installed on any PC that is running a Windows operating system.