In your image editor resize the image to the size you need it.
Resample the dpi to match the dpi you are engraving at (1000dpi)
Convert it to grey scale to get rid of the colour.
Convert it to black and white or 8 bit bitmap. ( depends on what the software calls it)
Choose half tone and tell it he number of dots to use
I prefer to use Corel Draw for all this. It's dead easy and you can select the type of dithering. ( floyd steinberg) seems to work pretty good.
Inverse the image for aluminium.
Note it might be worth working out your dot size based on the lens you are using for this then divide that into 1 inch.
Build log has an optics calculator for that here..http://buildlog.net/cnc_laser/laser_calcs.htm
I am a metric person so that would be something like 25/0.02. Isuspect you will find you can't get 1000 of your dots into an inch. Your steppers might not be able to increment by 1000th of an inch anyway. So, you are engraving at a resolution too high for your laser and losing money as you can get the same quality from a lower resolution and it's faster. You'll never see the difference.
Currently I suspect you are over engraving. Think of trying to draw a picture with your finger in lines by putting some ink on your finger and drawing a 1000 lines in an inch. You will get over lap.
Apparently our self build lasers max out on resolution at around 400- 500 dpi ( I am not sure of the truth in this )
Once you have worked out your maximum resolution do all the above and...
Change the scan gap as close you can to match the maximum resultion your laser can do.
For instance if your laser dot size max resolution is 172 dpi use the scan gap for 175 dpi.
Before you do this you might want to make a small black box bitmap and engrave at a really wide scan gap so you can see the lines.
If they don't match up you need measure the difference go into the settings and adjust the engraving backlash offset ( I think it's called that )
try engraving again.
This page is great for explaining this it appears to be down at the moment tho!http://www.engraversnetwork.com/support ... ppi-laser/
The invoice is in the post I probably saved you quite a bit of money in the long run ha ha.