You need to have a separate file for your signature. Your image file is not your signature file.
1. Open a brand new file, on the top menu, go to "File:New", then make sure the measurement is on "pixel". Set the size, for instance, I like my proportion to be width = 900, height = 300 or w = 750 and h = 250 (Idk what John uses ... w = 500, h = 200 maybe?). That is your template for your signature. If you see on the right hand side dock, you see that you only have one layer, which is background layer. I'd leave this alone and don't use it until you need to put anything in there. Layer usually is defaulted as 'transparent' (but you can always pick 'white' or others), but I would always want it to be transparent, so that whatever I put above that layer will show through.
2. Then go back to your pirate image file. Make sure your cursor is on Rectangle Select Tool. Draw a window across your pirate image, then right-click on your mouse and pick "Edit:Copy" or you can use menu on top "Edit:Copy". And go to your signature file. Before you paste your pirate image in there, create a brand new layer. Go to the right tool dock, and there is an icon (far left in the middle, looks like a piece of paper) that says 'create layer', click on it. Or you can go to the top menu "Layer:New Layer". Then you can paste your pirate image by doing right-click "Edit: Paste".
3. Remember, you always want to assign a different layer for each entity that you want to put on your signature. For instance, if you want to put two different pirate images, do them in separate layer. Text in separate layer, background in separate layer. Basically you need to open a new layer each time you want to add something to your template. Why? So that each entity would be independent and you can move it around and place it whereever you want it to be.
4. What kind of background do you want to use? If you want to use a screenshot from the game, you need to open the screenshot file in GIMP. Crop it to approximately the same proportion or size of your sig. template. You'll be surprised that you may have to resize your screenshot because it might be too big compared to your sig. template. But here's what you need to do: Open the screenshot file, then while your cursor is on Rectangle Select tool, draw a box over the part of screenshot that you want to be your sig. background. Then right-click "Image:Crop to selection". It will remove the excess parts of screenshot. Then right-click "Edit:Copy". Then go back to your sig. file, create a brand new layer (you will have to name each layer every time you create a brand new layer. I'd name it whatever image I have, i.e.: bg port royal, etc.). And then paste your background by doing right-click "Edit: Paste".
5. You can modify your background before you copy and paste it to your sig. template. If you want to modify, after you crop it, go to menu on top "Color:Hue-saturation". A dialog box will pop up and you can move around the little handle until you see the color that you like. If it's too dark or light, go to menu on top "Color:Brightness-Contrast". If you want it to be black and white, go to menu "Image:Mode-grayscale". The sky is the limit as how your background would look.