Placement of images keeps changing

Question

I am having a lot of trouble placing images where I want them in my document. With the first 3 images the it stops working. I am new to latex and still struggling to learn it. I have included the initial commands too.

\documentclass[letterpaper,11pt]{article}
\usepackage{graphicx}

The code I used to display the image where I want it

\begin{figure}[h]
    \centering
    \includegraphics[width=15cm]{all_sky_map.png}
    \caption{All sky map}
    \label{fig:all sky map}
\end{figure}
This is the image

This bit of code does not work and does not display the image where I want it
\subsection{Producing peaks in a spectrum}
\begin{figure}[h]
    \centering
    \includegraphics{peaks_spectrum_1.png}
    \caption{Peaks in HI 21-cm Spectrum at (26.76, -1.54)}
    \label{fig:peaks spectrum}
\end{figure}

and this is the image used in the second bit of code

Can someone help me?