Drawing on an image

I have a comic book in the format of a pdf. I would like to scanlate it. How do I draw polygonal objects on the bubbles in the comic book. I tried enabling gridlines but I can not draw on the image. I have to first move the image and draw it behind the image. I also tried sending the image backwards but to no avail. I can currently draw rectangular boxes and fill it with text but I want to draw objects by using the outlines of the bubbles to better match the bubbles.

Would you mind to explain the term “scanlate”? I cannot find it in my dictionaries.
Suppose you opened a pdf with ‘Draw’. What tool did you choose from the drawing toolbar?
Suppose you wanted to draw in front of (Z-order) a shape created for some content of the pdf. What type of shape? Did you make sure that shape was not selected?
Describe your issue to more detail. I cannot reproduce it based on the given information.

(I found info about “scanlate” meanwile.)

First you can open LibreOffice Draw 5.x, then 'File → ‘Open → Your.pdf’ which will be imported in multiple pages. You then may have to adjust the page sizes.
To draw a filled polygon onto the image can be a problem (the image becomes selected), until you figure out that you have to Click+Drag the first point (not necessary to see any canvas) and finish via ‘Enter’. Filled polygon is in a dropdown menu originating beside the ‘Free line’-tool (pencil icon).
After adjusting the colour/line of the polygon, you can insert a text-area to float on top of your talk-bubble.
A quick example here: http://sv1.fellsnet.is/libreoffice/Draw-PDF-ComicBubble.png

Have I understood you correctly?

Yes, this is exactly what I wanted

I’m not sure if LibreOffice is the right kind of software for your project. You want to look for more specialized dtp or draw software. I would choose a dtp application like Scribus, where you can put your pdf image in a layer you can lock, then make the translation in a separate layer. This way you also can put multiple translations in one document.

Alternative you could use a draw application, but most of those are restricted to one drawing per file, so you have to make as many files as there are pages. Later you have to put those pages together in some other software, which means double the trouble.

Only professional illustration programs like Adobe Illustrator have the posibillity of multiple pages in one document. But still it is a drwing tool and you are looking for a publishing tool.

LibreOffice cannot open pdf as pdf. It converts any pdf you try to open to a ‘Draw’ document. Layers and locking of layers are also available in ‘Draw’. Pdf inserted as an image into a ‘Writer’ document should not be the way to do the “scanlation”. Depending on the software used for scanning, on the languages and kind of writing used in “the book”, and on additional aspects, very different situations are possible.

Basically I want to draw a polygon matching any bubble within the image. I can do so but only when I move the image and see the clear whitespace. I want to draw on the image so that I can draw correctly.

I already told you that I cannot reproduce you issue. In other words: I can well draw shapes in front of an image as long as it isn’t selected. I would need more (better) information to be more specific. I still don’t know if you try to work in ‘Draw’ or in ‘Writer’. What format is your “image”? (Yo talked of pdf.) Sometimes LibO version and OS are relevant …

@Henk1: LibreOffice Draw 5.x can open multipage PDF’s and keep all the pages intact in one file. Work on that one and export again as multipage PDF.