If your Watermark is not being applied to the images you post with Postcron, you might want to check your Watermark Settings for 3 preferences:
Applying
When setting up your Watermark, you might have missed the Apply by default option. In this case, the Watermark will only be applied to your images if you select the Watermark option on the Composer. If you’d like to have the watermark applied to all the images you share with Postcron, make sure you enable the Apply by default option.
Size
When you set up your Watermark, you can choose its size. It can cover the whole image, or be so tiny that it won’t be visible. You can change your Watermark’s size by moving the Size slider. Postcron will show you a preview so you can see how it will look on your post.
Transparency
When you set up your Watermark, you can choose its level of transparency. It could be 100% solid/opaque or totally transparent, in which case it won’t be visible. You can change your Watermark’s transparency level by moving the Transparency slider. Postcron will show you a preview so you can see how it will look on your post.