There is the way to upload an image from the PC into Instagram without installing additional applications or browser extensions. At least, it works with Google Chrome browser.
Instruction:
Step 1. Open your instagram page. Click the right mouse button on any place on the page and choose "Inspect" in menu.
Alternative step: press buttons Ctrl + Shift + I simultaneously. This combination should open the same panels by default.
Step 2. After previous step you should see the several panels for web developers with codes etc. But they are of no interest to us now. The only thing which we need is the button with two squares on it (I suppose that they symbolise smartphone and tablet). If you will point mouse cursor on it, you will see the hint "Toggle device toolbar". You should click this button.
Then point the cursor to the page. If it is turned into circle, than everything is ok: it means that we look at the page in regime of smartphone. If the cursor doesn't turn into circle when you point it on page, try to click the button with squares once more.
Step 3. If the regime was toggled successfully (mouse cursor turns into circle when you point it to your page), then refresh your page (for example, with F5). After that buttons, available for mobile version of Instagram, should become visible to you:
Step 4. All what's left is upload the image with plus button, as we do it on smartphone.
I don't know how to upload multiple images at once, in one post. I think that it is impossible and every instagram post may contain only one image? 