I'm sorry if this is a very basic task, but I'm fail to find one for Inkscape.
I want to make this logo a watermark. I need to make it transparent enough. How do I do this? The sphere is a bipmap image, the text is vector.
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Make a group which contains your bitmap and vector parts. Give to it the wanted transparency in the objects panel.
In the left your attached photo and a black rectangle are both selected and Ctrl+G is pressed to get a group.
In the right there's a duplicate of the group. It's selected and it's opacity is reduced in the Objects panel.
NOT ASKED: For dark works you need also a negative where at least black areas are turned to white:
This watermark is your attached image where the full black and full white areas are both inverted.
In practice you'll need watermarks only for low resolution rasterized images of you works. I guess you do not let people have access to your vector or high resolution drawings. IrfanView and many other programs can watermark bitmap images as a batch job.
There are a couple of ways to do this. As the comment to your question says, you might decide to use a bitmap editor but that would mean you can't so easily tweak the transparency level within Inkscape as you work. Since they seem not to know how to do what you ask then it isn't such as basic question!
Fortunately though it is very easy.
If you select the bitmap in Inkscape and open the Fill/Stroke dialog box you will see that it has an 'opacity' slider at the bottom. Making this less than 100% will make it partially transparent. This is a whole-object effect and works even on bitmaps.
(You will see 'alpha' sliders below the RGB sliders at the top of Fill/Stroke dialog, that affect the transparency of the fill and stroke colours seperately, but which don't work on bitmaps.)
Hope this helps.