Step 1, making the screenshot
On Android, press the power off/on button and volume down at the same time.
On iOS, press the power off/on button and the home button at the same time.
Step 2, getting the screenshot onto your computer
Then either through USB or through email get your screenshot on your computer.
Step 3, resizing the screenshot
When you upload an image that has a very large resolution, for example the resolution of current smartphones, the forum will try to display it at full size, but cut off the preview because it's so big. To prevent this, you'll want to open the screenshot in imaging editing software (Microsoft Paint, GIMP or other free software will do) and resize it to ~30%.
Step 4, uploading the screenshot
Then, on the forums, when typing a message, under the "Submit" button there are some options. There is also the "Upload attachment" tab, where you can navigate to your image on your computer and upload it.
After uploading, you can let the forum put your images at the end of the post, or you can choose to place them inline, if you want to tell a story accompanied by screenshots.