Technical problems

To obtain a digitized copy of a drawing you should not use the mobile phone, it is best to use a scanner and a program like Photoshop

There is no sharp focus and it is very noisy.  It looks like you took this photo with your phone's camera.  While not impossible to get photos good enough for stock on your phone, it is very difficult to do so if the lighting conditions are not ideal.  It has to do with the very small sensor size of the camera on your phone and the amount of automatic processing done by the phone. 

On a side note, this looks like an original piece of artwork that would also require a property release.  If you are the artist, you would still need to complete a property release form.

If you are new to stock photography, take a look at some of these resources:

https://helpx.adobe.com/stock/contributor/help/photography-illustrations.html

https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html