Keep in mind a couple things when posting pics on the web:
Keep file size on the smaller size so that it uploads and downloads faster, and occupies less space. There are two ways to do this, resize the picture, or decrease the resolution. When you take a pic on your digital camera, the size is large and the resolution is high because you want high quality for printing. On the net, you don't anywhere near that high of resolution or size. I use a free program I downloaded off the web called Irfanview. It has an Image>resize command. I reduce it to the smallest standard size on there. Generally can get the file down to about 60KB, which is good for posting. Remember that when resizing, to keep the picture to a size that will fit on other peoples screen without them having to scroll to see it all.
So see how big your file size is, put your cursor over the picture file icon, right click and select properties.
Once you have sized your pic files, upload them to a picture site for hosting. I use picturetrail.com for about $20-30 a year. I think there are some free sites too, but I liked the picture trail options.
Once you have it hosted, simply open the picture, put your cursor over it and right click. You'll see the web address for the pic, copy that (highlight it and right click >copy). Then when you are in the forum post you can paste the address as a link, or use the forums image tags or link to past the address to post it as a picture instead of a link.
After you do this a couple of times, you'll see it's pretty easy. I like to up load multiple pics at once to save time. Happy posting.

If you resize the photo to something like 800x600 it comes out the right size.

And you don't have to slide the scroll bar left and right to see the whole picture.
The sites you store pictures on usually recommend that pictures are saved at about 800x600 for best Internet viewing.