Firstly you'll want to make an account on photobucket or similar, I'll be using photobucket for this

When you log in you'll be granted with this screen,


Once you've clicked the button to upload you'll be granted with this



Select what pictures/videos etc you want to upload, hold ctrl to select more than one at once



Once you've clicked ok it'll start uploading, once it's done wait for the page to reload



Once they're uploaded and the page refreshed your images will appear at the top of your list



For posting on forums you want to select the image code, it begins with [img] and is nicely coloured in green here, it should autocopy it for you..



go to where you want to post and press Paste, you'll see the link of the imge in a pair of image tags, ([img])

Like the one highlighted in green

When trying to post images, I keep getting the "It was not possible to determine the dimensions of the image." error message.

alright, thanks

Also, if you get that message, it should return you to the post reply window... simply hit 'submit' again and all should be fine.