Only the U.S. Sponsor's income is considered when determining if you meet the income requirement.
The Fiancee Visa and the Spouse Visa do not have the same income requirement. Applicants for a a Fiancee Visa will need to show that their U.S. Sponsor's income is 100 percent of the federal poverty guideline. Applicants for a a Spouse Visa will need to show that their U.S. Sponsor's income is 125 percent of the federal poverty guideline.
For a Fiancee Visa a couple with no dependent children has an income requirement of $15,510 per year.
For a Spouse Visa a couple with no dependent children has an income requirement of $19,387 per year.
For the 48 Contiguous States.
Although they want to see your last income tax return that you filed they don't qualify your income by that. You will need to show them pay-stubs from your job to prove how much you are making at the time of her interview at the embassy. This is the income they will use to make sure you meet the income requirement so even if you were short on your last tax return that's alright as long as you have the income at the time of her interview.
If you are self-employed you will need to prove your income and your tax return becomes much more important than someone working for a compnay as a regular employee. They will also want to see your bank statements.
When assets are used, the assets must equal 3 times the difference between the annual income and the needed 100% (100% Fiancee Visa, 125% for Spouse Visa) of the poverty level. For instance, if you needed $15,510 income and you had an annual income of $13,510, you would be short $2,000. You would need 3 x $2,000 = $6,000 in assets in addition to your income.
In general, if you are deficient on yearly income for sponsorship and your assets are somewhat borderline, do not take chances--have a co-sponsor (Only allowed for a Spouse Visa). If you do not have a job or a steady income from other sources (such as retirement income), you will likely have to get a co-sponsor for your spouse unless your assets are adequate.