Zoom all the way out (so that it's 1:1
again)

Go
to layers palette and click on
"layer 2" to activate it
Flood fill with a light colour that
isn't part of the background or image,
for example, if your background was
green and the outline of the image is
black, flood fill in white

Go
to the layers pallette and click on
"layer 1" to activate it
Using
your eraser tool at the following
settings:

Round
shape, size 10, hardness 100, opacity 100,
step 25, density 100. Don't build
up.
Zoom
in once and take off any obvious bits
that show up on the white, making
sure you don't go over any of the edges
Before

After

Save
your work
Zoom in
twice and use the eraser tool at the
following settings:

Round
shape, size 5, hardness 100, opacity 100,
step 25, density 100. Don't build
up.
Take off any obvious
bits making
sure you don't go over the edges
Save
your work
Zoom in
as close as is comfortable for your
eyes and use the eraser tool at the
following settings:
Round
shape, size 1, hardness 100, opacity 100,
step 25, density 100. Don't build
up.
Take off any obvious
bits, making sure you don't go over the
edges
Save
your work
Zoom
all the way out again
In
the layer pallette, right click on
"layer 2" and choose delete

You
should now have something that looks
similar to this:

Go
to file --> export --> picture
tube

Name
your tube and hit OK
Crack open something fizzy, sit back
and relax - you've finished!!