It is a good idea to trim your images down to a decent size before posting them online.
A Little Bit of Theory
1. The size in terms of pixel size: Each image will have certain dimensions measured in pixels, such as 640x480 px. You can get an idea of what's appropriate by considering that the most common screen resolutions people use are 800x600 px and 1024x768 px. It is considerate if you keep your images in such sizes that people don't need to scroll horizontally to view them.
How do you know the size of an image? In modern versions of Windows, you should be able to figure that out just by selecting the image; the details, including a thumbnail and the pixel size, should be displayed in a separate column on the left side of the folder view.
2. The size in terms of kilobytes: To keep even those with slower internet connections happy, it is polite to keep the images you post online down to a decent size. There is no strictly defined limit for a decent size, but generally it is not a good idea to post an image which is 400 KB or 800 KB in size.
The size of an image can be shrunk down by a) shrinking the image dimensions, for example shrinking a 1920x1440 px sized image down to 640x480 px, and b) by selecting the appropriate JPG compression, which can be chosen in most image manipulation programs. A quality of 50 to 60 (or 5 to 6 out of 10) is generally sufficient for the internet.
Getting the Job Done
Then, we need something to do the trick with. There are plenty of freeware applications for this, you can browse around at Download.Com (link leads to freeware image editors) and pick your choice. The following instructions are written for Easy Thumbnails 2.8, a handy, free tool available for download here: http://www.fookes.com/ezthumbs/
1. Download, install and start the program.
2. Select the desired image by browsing your way to it on the left side of the screen. Select it.
3. At bottom left under the File-section, select the folder where you wish to have the image saved.
4. Under the Settings-section, set max. width and height to 640 px. Set JPEG quality to 50 or 60. This section also allows you to adjust the brightness, contrast, sharpness etc. of the image. You can preview the image you are about to create by switching between the Original Image and Preview Output views.
5. Click the Make-button at the bottom right of the screen to produce the resized image.
You can, of course, adjust the settings above as necessary, but the settings above should give you a reasonable image you can post in the forums.