First you have to put a line like
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image 18,"sys\gfx\if_values_black.bmp",0,0;
18 is just an image-ID... see image if you understand some german (maybe you can translate it with google, on www.dict.cc or something)
Then you have to put the progress bar over the background but under the "if_values.bmp" (therefore it's important to set the right IDs...^). Something like that
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image 19,"sys\gfx\progress_o2.bmp",$s2lis_oxygen-95,31;
I took the variable "$s2lis_oxygen" which represents the current oxygen reserve and decreases every two seconds, but that isn't that much important for you now.
You'll have to decide for youself what you put in there, and also you'll have to put in your graphics ("progress_o2low.bmp" in my case). Maybe the x- and y-coordinates a differing a little bit too, depending on your used graphics.
The ID 19 means that this bar will be set before the black background graphics but behind the if_values.bmp.
So for example my oxygen level is at 100 percent. Then $s2lis_oxygen-95 gives 5 - the bar reaches to the end...
If my oxygen level is at 0 percent then $s2lis_oxygen-95 gives -95 - the bar will not be seen. Hope these examples explains enough (maybe you even don't need such examples ).
Hoping I did not forgot anything - 'cause I'm right tired now. But if you're having any questions just post it here.
Maybe I should upload my graphics as an example: <download>
If you have understood this you'll see that the principle itself of course is quite simple.