You might be running into CQ00312586; 'Changing the TOC layer name renders the list of available photo fields (in Check Out Wizard) empty' which is actively being investigated by ArcPad team. Possible workaround is to change the layer name back to original name and re-add to ArcMap map document, then perform data check out to ArcPad and the photo fields are available as expected.
If this is the behavior you are experiencing, please contact Esri Support Services via Esri Support site links for phone, email, webform, or chat http://support.esri.com/en to have your information added to the CQ. If you are experiencing other behavior and have current ArcPad maintenance, please feel free to submit your question to help troubleshoot this issue. Please include ArcPad version, service pack or build information and provide screenshots of errors or dialogs which illustrate behavior you are reporting.
Thank you,
Denise
The two relevant tracking numbers in our system are as follows
CQ00312811 'Potential picture fields do not appear in Select Picture Options if layer name contains a space or underscore'
CQ00312586; 'Changing the TOC layer name renders the list of available photo fields (in Check Out Wizard) empty'
Both of these software defects refer to the behavior that the list of available photo fields is not populated when the layer name in ArcMap's table of contents doesn't match the feature class name in the geodatabase.
Underscores in the layer name are not problematic if they also appear in the feature class name, but any special characters (points, spaces, etc.) that are not supported in the feature class name are definitely going to cause problems. Even supplying different text for the layer name in the Table of Contents without spaces will cause the behavior.
The ArcPad Team is aware of the problem and working to resolve it. Luckily, the inability to specify specific fields when checking out data doesn't keep users from taking pictures in the field and associating them with the appropriate field. If you have any additional questions, please don't hesitate to contact Esri Support Services.