A dummy is only required (or wanted) for an illustrated picture book submission where you are the author and the illustrator. If you are author-only, you submit text-only.
If you are a graphic novelist (and doing both the writing and illustrations) you would do a proposal in PDF form, which includes an overview of the project, a bio, a synopsis, sketches for a chunk of pages, and a few sample pages of full art, along with links to your portfolio, etc. If you are an author-only, you submit text-only.
If you are a writer, writing a MG or YA novel in which the illustrations are decorative (and you are not the illustrator) – such as LEVIATHAN or the Elf King books you describe – you query exactly like a regular novel, no difference whatsoever, except you say “I envision this to have illustrations, in the vein of LEVIATHAN or the Elf King books” etc. The publisher would hire the illustrator.
If you are an author-illustrator, writing a MG or YA novel in which the illustrations are decorative, query just like a regular novel, but link to your portfolio, and it’s fine if you also submit a PDF with some samples, and a few pages laid out with art, but not strictly necessary.
If you are a writer, writing a MG or YA novel in which the illustrations are hugely important (but you are not the illustrator) – like say you are writing a novel about a kid who draws cartoons and processes their emotions through cartoons, you query exactly like a regular novel, text-only, but you say in the query, “My novel is about a kid who draws cartoons which express their emotions, so portions of this ms are written in graphic novel format.” Then you describe the illustrations in the text, for example
paragraph of text
paragraph of text
[four graphic novel panels: panel one: thing that happens, panel two: response. panel three: whatever is next. panel four: you get the idea.]
paragraph of text
If you are an author-illustrator, writing a MG or YA novel in which the illustrations are hugely important, you’d query like a novel as in the above, but also make a PDF like the graphic novelist one.
If you are an author-illustrator, writing a MG or YA novel in which the illustrations are decorative, query just like a regular novel, but link to your portfolio, and it’s fine if you also submit a PDF with some samples, and a few pages laid out with art, but not strictly necessary.
If you are an author-illustrator, writing a MG or YA novel in which the illustrations are hugely important, you’d query like a novel but also make a PDF like the graphic novelist one.