cover not displayed

[ffarr]

Hi everyone,
i post my cover image with this code:

<img alt="" src="../Images/cover.jpg" />

It's in the first page of my ePub. I added the semantic "cover image". I added the semantic "cover" to the file called cover.xhtml.

When i open it in Sigil, i see the image, but if i open it using Adobe Digital Editions (which i'd like to set as recommended reader) i just see a blank white page. I cannot get why.

I created another ePub in Sigil and i think i did exactly the same, and i can see the cover.

Am i doing something wrong with the image? It's a simply .jpg ..

Thanks to you all