This is how my flow looks like. I left the image field blank, then i created the custom actions for the modify and add new device. I tested the catalog item. It didn't add the image to the table or modify it. I'm not sure where it went wrong. 

Trigger: Service Catalog

Action:
1. Get Catalog Variables
2 If the request is modify the device
1. Modify name
OS
Model:
Type:
Image = Blank

Custom Action: action to set image

3. If the request is add new device
1. Add name
OS
Model:
Type:
Image = Blank
Custom Action: Add action to set image

This is my custom action looks like 

(function execute(inputs, outputs) {
var sourceAttachmentId = inputs.attachment_sys_id;
    var targetSysId = inputs.target_sys_id;
    
    //gs.info('Attachment ID: ' + sourceAttachmentId);
    //gs.info('Target Sys ID: ' + targetSysId);
    
    var sourceAtt = new GlideRecord('sys_attachment');
    if (!sourceAtt.get(sourceAttachmentId))
        return;

    // Validate that it is actually an image
    if (!sourceAtt.getValue('content_type').startsWith('image/'))
        return;

    var target = new GlideRecord('u_am_device');
    if (!target.get(targetSysId))
        return;

    var gsa = new GlideSysAttachment();

    // Copy the actual binary content
    var newAttachmentId = gsa.writeContentStream(
        target,
        'image',
        sourceAtt.getValue('content_type'),
        gsa.getContentStream(sourceAttachmentId)
    );

    // Convert the normal attachment into an Image-field attachment
    var newAtt = new GlideRecord('sys_attachment');
    if (newAtt.get(newAttachmentId)) {
        newAtt.setValue('table_name', 'u_am_device');
        newAtt.setValue('table_sys_id', targetSysId);
        newAtt.setValue('file_name', 'image');
        newAtt.update();

        target.setValue('image', newAttachmentId);
        target.update();
    }
})(inputs, outputs);