Just go to Layer - Transparency - Remove Alpha Channel if its on that then click the Add Alpha Channel.
Try saving the image into your 'Pictures/Images' folder and then return back to GIMP; select 'File' in the top left and 'Open as layer'. That should then lead to your documents. Find where you saved the image and open it up.
Tip: When saving your image, something like this MIGHT pop-up. If so, click what has been selected in this example:
(http://i48BannedImageSite/3353ytv.png)
It should then go onto a page where you see the image's original size.
You then need save your drawing on a Transparent Background by making sure all the layers do not have Alpha Channels. You do this by right clicking the layer's canvas/select layer [on the toolbar], then select 'Transparency' and Remove the Alpha Channel if it hasn't been done already [Make sure that all you can see in the background are greyscaled squares - also try using the eraser tool on a coloured/blank background if you don't.]. Once you've done that save your drawing as a .PNG and if it tells you to 'Merge' the layer, do so. Don't flatten it. [Don't close GIMP down after]
Finally go to File >> Open and find the Forest background you had saved in your Images. When you save an Image, it seems to become larger instead of Copy and Pasting. Once the Image is opened, go onto the canvas with the Forest background and select 'File' again and 'Open as layers' then find your wolf and place it to how you want it on the page.
I hope I made this clear enough for you, if you're still having trouble, just let us know.