I'm trying to use the anchor point of an image, that is large two entire pages, to anchor it at the text that it is in the precedent pages, like in this image. The problem is that when I try to do that, the image goes to that page, as a background to the text. How can I do to anchor the image but mantain it in the position that it is actually? I need to do that because I'm trying to export the book as an EPUB.
I'm not entirely sure it will work as intended, but you could try to give the image a text wrap. Select it, and find the
Text Wrap palette through Window > Text Wrap or Alt+Ctrl/Option+W. Click the second icon 'wrap around bounding box'.
That way, the image will not tolerate any text on top of it, pushing any text that would come under or over it to lower on the page, or to the next page (provided you have your text frames linked).
While Vincent's answer is correct if this image were to be on a single page to be placed inside a text block I think you're going to run into issues on the export because you will have to export the ePub as a fixed layout to get this to work as a spread on a device.
Since you will run into complications with the text and the image I would use what's called Text threading. There is a great example article on this over at Layers called: "Mastering Text Threading in InDesign". I mention text threading because if your intent is to be a reflowable ePub you will have to approach the text in this manner so that it is used in it's designed state, which is to reflow.
The linked question shows you how you can take multiple text frames and link them around an image, example:
Even though this will work be aware that when you go to export as an ePub if you choose reflowable it will not follow the structure you want and will be thrown off. As stated, there are two ways to correct this. Code the page as one single page spread in fixed layout with all the other pages as reflowable or skip InDesign and try to modify the CSS to work on rotation between Portrait and Landscape.