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 imageeapi 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 "";

$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"
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 may 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.
$attname = "dc:creator"; return @reset($file_metadata->$attname);
See the documentation for the meta suite and HOOK_metadata_from_file() for more about this data structure..