How to add image field to module install file
You are reading the wrong documentation. Please read Data Types instead. There is no image type for database field.

The available data type are:


serial
int
float
numeric
varchar
char
text
blob


Since Drupal manage file (including image files) in file_managed table with key fid. What you need is to create a int column for storing the value of foreign key fid. The column definition should be:

'image_fid' =&gt; array(
  'description' =&gt; 'The {file_managed}.fid being referenced in this field.',
  'type' =&gt; 'int',
  'not null' =&gt; FALSE,
  'unsigned' =&gt; TRUE,
),