This is the First map I have made.

[Jacktannery]

@Jalyha: You should export the map, gimp's xcf is not an actual image, but gimp's impression of it.
Go to File > Export (or shortcut Ctrl+E) and save your map as something.png. And now you have a displayable version of your map with original quality.

Alternatively, to reduce the size, before exporting go to Image > Scale Image and change its size. Just be careful not to save it after scaling, just exporting, otherwise you will permanently reduce your map's resolution.

You should Export to a .jpg (not a .png) and go with default compression (usually 85-95%) for the purposes of uploading here or on other places on the internet. This will make the resulting image file about ten times smaller with no loss of quality visible to the naked eye. When you Export it as a .jpg the next dialogue box has an optional box called 'preview in other window' or something like that - if you click the box yes, then after a few seconds it will show you the size of the exported image. If this size is above the forum limit (c. 4.5MB) you can decrease the compression percentage to say 60% or whatever until the image size is small enough to post here. In this way you can get the best and biggest export of your map that would fit on these forums quite easily.