I don't think there is a guide on this and i have seen a fair few people asking how to take a screenshot so i am going to show you what to do
You need to create a screenshot folder you can do this by going to Documents > Sports Interactive > Football Manager 2010 and then create a new folder called screenshots.
When you are on the game and you are on the screen you want to take a screenshot of press ALT + F9 and then that will take the screenshot and that will now be in your screenshot folder
Step 3 Uploading the screenshot onto FM-Base
When you are posting you will see a paper clip icon.? All you do is click that icon and then click Choose then you will need to find the screenshot and that is in the folder you have just created so go to that folder and double click on the image you can upload 6 images at a time but if you do decide to upload more it will take longer to upload so you have decided how many you want to upload press Upload it will now say Uploading File(s) - Please Wait the upload will be complete once that has gone then all you have to do is post and the screenshots will appear on your post
BTW - If you don't want to go to Documents > Sports Interactive > Football Manager 2010 all the time if you right click in the screenshot folder and select create shortcut you will get a new file which you can place anywhere you want and they will also appear in there when you press ALT + F9