A GIMP tutorial for aimeekim... :-)

Step 1.

Choose the image you wanted to cut-off...

Step 2.

After choosing the image, click the "Path Tool" icon on the left tool box...

Step 3. 

Start plotting to image, make sure it's well plotted...

Step 4. 

After plotting, click the "Selection Path" button on the lower left toolbox...

Step 5. 

Right click then you'll see a box, look for "Layer" then another box will automatically open...

Step 6. 

Once the another box opened, look for "Mask", another box will automatically open, look for "Add layer mask"

Step 7.

Once you click the "Add layer mask", an independent box will open, make sure to choose "Selection" then click "Add"...

Step 8.

Ta-Da! You're finally done, you can now save/export it to a PNG image...

You can directly copy and paste it, just go to "Edit" > "Copy Visible", then moved to the main poster and go to "Edit" > "Paste as" > "New layer"

I hope you get what I mean, actually I'm not good in making tutorials since I'm also learning, but I hope this helps...