Hi Dario,
I`m not sure I get what you mean here. Do you mean you want to insert alt tags inside blog post cover, or blog image?
http://take.ms/uT9RS
If not, perhaps you can provide some screenshot with explanations to clarify this to us? Let us know, thanks.

Hello Raymond,
I send some screenshot to explain my question

Hi Dario,
Unfortunately, this is not possible.
Alternatively, you can try using a photo editing tool like Photoshop to edit the image beforehand yourself before uploading it to your blog post.