New Post: Microsoft Access 2007 Application.GlobalCallback not triggering/ program freezing

Here is another tip: try to pass extents which are slightly different from current map extents (requests with same extents may be considered as duplicated and dropped):

Set myImage = axMap.SnapShot3(axMap.extents.xMin - 1, axMap.extents.xMax + 1, axMap.extents.yMax + 1, axMap.extents.yMin - 1, axMap.width * 2)

This solved the issue with 256px width for me (I had it to). But I have no idea about 512px issue (naturally debugger reports that map width is 512 in that case and all works well).
axMap1.TilesLoaded += axMap1_TilesLoaded is C# syntax to attach event handler, your VBA code already doing it. Also no need to create image and settings sampling mode before snapshot call, a new instance will be returned all the same.

Hope it'll help,
Sergei