I'd like to add a few things to the above.
I'm personally not even familiar with Civ 5's specific method for doing screenshots, I still use
Irfanview, which is a simple freeware program which can be used for taking screenshots as well as editing them.
Examples of editing is making a crop; draw a square and choose "Crop selection" (under the 'edit' tab).
Or you can use "Resize/Resample" under the 'image' tab to make your whole image smaller.
Making images smaller has the main purpose of letting it better fit on the forum's pages. 900 pixels is a width that'll still look good on the page, but your screenshots will be bigger than that. You can either crop or resize, or, if you don't want to do that, you can use spoiler tags, as Artingel78 has done above.
Adding text to your image is also possible, under the 'edit' tag you'll find "Insert text into selection". The 'selection' is a square you need to draw first on your image for the text to be put into. All types of fonts and sizes are possible under 'choose font'.
If you want to place dots on a picture of a map to show what you think are good city sites, It's easiest to bring up the built in Paint program, under the 'edit' tab is "Show Paint dialog". In this dialog you can choose a 'Width' of 40 pixels or so, then with freehand you can just tip somewhere on your image to make a huge dot. Clicking underneath 'Fill' on the Paint dialog brings up a window with colours to choose from, if you're making several dots it's handy to make them different colours, so you can speak about 'yellow dot', 'blue dot' and such and it's clear which dot you're talking about.
Save your images as JPEG's, they don't take many kb's. They may be BMP's while you're working on them, but upload them only as JPEG's, that's much more convenient.
For showing an image on the forum's pages it's often handiest to upload it first at an image sharing site like
Photobucket. The options Artinglel78 mentioned are just as good I suppose, except personally I'm not familiar with them.
Each image you're uploading will get its own url, so that's what you link to. Use the image tags for that; on Civfanatics, when you're posting, choose the picture postcard symbol for bringing up the image tags.
It's not always needed to use an image sharing site to show your pictures here. You can also use the 'Manage Attachment' function under 'additional options' you'll find when you're on the 'reply to thread' page.
I haven't used that for years myself, what I believe it does is just putting a thumbnail of your image underneath your post.
The 'preview post' function is very handy for checking whether everything looks the way you had intended, with all links working.
