When rendering text on an image, enter PHP code that will return your dynamic text. Do not use <?php ?> tags.
EG
return format_date(time());
return $file_data->description ? $file_data->description : $node->teaser;

Note that executing incorrect PHP-code can break your Drupal site.

Available data objects

Many different types of data derived from the image may be available as php values:

$node
A handle on the owning node object, if any.
return $node->title;
return format_date($node->created);
$image
A handle on the internal imageapi technical image object.
return $image->source;
return basename($image->source);
return $image->info["filesize"];
$file_metadata
A collection of metadata that may have been deduced from other sources or context. Values inside $file_data is namespaced, structured and attributed, so can be complex to read.
drupal_set_message('<pre>'. print_r($file_metadata,1). '</pre>'); return ""; ... will display the structure.
EG:
return join(',', $file_metadata['dc:subject']); ... will retrieve all tags found embedded in the picture itself.
$file_data
A simplified list of metadata values that may have been deduced from other sources. This is a flattened, simplified version of $file_data with no namespaces and all-lowercase attribute names.
$file_metadata->Iptc4xmpCore:Scene['pjmt:0'] = "exterior view"; becomes $file_data->scene = "exterior view"
EG:
return $file_attributes->description
return $file_attributes->copyright

Where the data comes from

If it's an image.module image then a $node object with its values should be available.

If it's an image that has been attached to a node using CCK filefield imagefield (or just filefield) then as well as the parent $node object, the $file_data object that may contain a file description (or other meta supplied by cck) from that file field.
return $file_data->description;
So far that seems to be the only available 'data' provided by filefield, but you can investigate the node structure using devel.module or print_r() to see what else this array actually contains.

If it's a file that's just been attached using upload.module, a $file_data object may also have a description.
return $file_data->description;

If the image path is detected as belonging to more than one node, just the data for the first one found is returned.

If you have meta_inspector available, then many more (namespaced) metadata fields may be available on the $file_metadata and $file_data object. Note that they will often be structured arrays.
The metadata inspector may try to scan for embedded (EXIF or XMP) data from the file itself, depending on the libraries available to you.
$attname = "dc:creator"; return @reset($file_metadata->$attname);
See the documentation for the meta suite and HOOK_metadata_from_file() for more about this data structure..