You can create an example file by downloading one of your images in .txt format. If you don’t have any images in your My Images gallery, view one of the Savable images in the Public Gallery (Savable images have a disc icon shown in their thumbnail) and click File->Save As to save a copy into your My Images gallery.
The .txt image format is tab-delimited and looks like this (where <tab> is a tab character, <Cr> is a carriage return character, and <Lf> is a linefeed character):
numCols<tab>numRows<tab>pixelSize (um)<Cr><Lf>
648<tab>484<tab>1.7465<Cr><Lf>
Z-data (um)<Cr><Lf>
14.9385<tab>14.9458<tab>14.9665<tab>…<Cr><Lf>
<Additional lines of height values with each row on a separate line>