Hi @Roman ,
I am able to reproduce this scenario and it seems to be by design. As per this documentation, we can see for image type web part we can only get text data in data property. Moreover in this documentation its mentioned that the sitePageData resource represents the properties of a webPart and as these properties vary by web part, this is an OpenType with no fixed properties.
Also, Please note that this API is under the /beta version in Microsoft Graph which is subject to change. Use of these APIs in production applications is not supported.
Hope this helps.
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